For instance, a device for suppressing resonance by an oscillating motion of an inertial mass is described in Japanese Patent Laid-Open No. 2002-340097. In a device taught by Japanese Patent Laid-Open No. 2002-340097, a pendulum mass is pivotally fixed to a radially outer side of a pulley by a pin, and a natural frequency of the pendulum mass governed by a distance between the pin and a gravity center thereof is adjusted to a number of torque pulses per rotation. The pendulum mass is oscillated by the torque pulse of the pulley as long as the pulley is rotated, and hence the pin and a pin hole are always subjected to high friction. In order to ease such friction between the hole and the pin, an inner surface of a pin hole is covered with a fluorocarbon resin film.
In turn, Japanese Patent Laid-Open No. 2011-504987 describes a force transmission device in which a torque converter is provided with a rotational speed adaptive absorber comprising a disk shaped rotary member rotated by torque, and an inertial mass attached to a radially outer side of the rotary member while being allowed to oscillate. The rotational speed adaptive absorber of this kind is immersed in the oil so that a friction site can be lubricated to reduce the abrasion thereof. However, a resistance of oscillating motion of the inertial mass is increased by the oil. According to the teachings of Japanese Patent Laid-Open No. 2011-504987, therefore, the rotational speed adaptive absorber is designed taking account of such oil resistance.
Japanese Patent Laid-Open 2001-153185 also describes a dynamic damper in which a friction site between a rolling inertial mass and an inner surface of a chamber housing the rolling mass is lubricated by oil. The dynamic damper taught by Japanese Patent Laid-Open 2001-153185 is used to damp a vibration of a crankshaft of the engine. For this application, a plurality of chambers are formed in a rotary disk fitted onto a crankshaft, and a radially outer section of an inner surface of the chamber serves as a raceway surface. When the hub is rotated together with the crank shaft, the mass held in the chamber is centrifugally pushed onto the raceway surface. In this situation, the mass is oscillated on the raceway surface by torque pulses applied to the crankshaft and the disk at a frequency of pulsation. According to the teachings of Japanese Patent Laid-Open 2001-153185, an oil hole is formed in the chamber to introduce oil in a crank chamber to lubricate a friction site between the inner surface of the chamber and the mass.
Thus, in a device for damping vibrations by oscillating the mass at a frequency of pulsation, a friction at the friction site between the mass and a contact portion has to be eased. To this end, given that the fluorocarbon resin coat is applied, abrasions at the friction site may be reduced. However, an additional work for forming the coating is required during a manufacturing process, and a number of components of the vibration damping device has to be increased.
Instead, according to the teachings of Japanese Patent Laid-Open No. 2011-504987 and Japanese Patent Laid-Open 2001-153185, oil is used to lubricate the friction site. In those cases, however, the inertial mass or the rolling mass has to be immersed into the oil and hence a resistance of the oscillating motion is increased by the oil. For this reason, the mass may not be allowed to be oscillated at desired frequency. As described, according to the teachings of Japanese Patent Laid-Open No. 2011-504987, the oscillating frequency of the rotational speed adaptive absorber is adjusted taking account of effects of oil. However, a viscosity of oil changes significantly depending on a temperature, and the effects of oil are changed depending on an amount of oil. Therefore, it is rather difficult to keep oscillating the mass at the desired frequency.